1. Field of the Invention
The present invention relates to an optical device for optical communication, and more particularly to a method for generating data light with high duty rate, a device thereof and applications thereof.
2. Description of the Related Art
Currently in an optical fiber communication system, a wavelength-division multiplexing (WDM) multiplexing signal light whose bit rate per wavelength channel is 10 Gb/s or 40 Gb/s in a wavelength domain is put into practical use. The WDM system is characterized in that communication capacity can be easily increased by increasing the number of wavelengths. However, if the number of wavelengths increases, it becomes difficult to individually monitor/control all wavelength channels. There is also an optical time-division multiplexing (OTDM) system extending capacity by multiplexing data light with narrow pulse width against a bit interval though its bit rate is 10 Gb/s or 40 Gb/s, that is, data light with high duty rate in a time domain, as a multiplexing method that strikingly contrasts with the WDM multiplexing.
In a hybrid system of OTDM multiplexing and WDM multiplexing methods, when some capacity is realized, the required number of wavelengths decreases as the multiplicity of the OTDM multiplexing method is improved. Therefore, in this case, the problem of wavelength channel monitor/control is mitigated. Data light with high-duty rate is required to improve the multiplicity, and it is also important for data light source for OTDM multiplexing method to stably output data light with bit rate that accurately meets the requirements of the system.
FIGS. 1 through 3 show the configurations of the prior art.
As shown in FIG. 1, a data light source that stably outputs data light with bit rate that accurately meets the requirements of the system generally modulates by data using an intensity modulator 10. In the configuration where continuous light (CW) outputted from a CW light source 11 is modulated by data using the intensity modulator 10 driven by an electrical data signal, data light can be always generated in synchronization with the bit rate of a system. However, its duty rate is approximately 2, and it must be improved.
As shown in FIG. 2, although there is a configuration using an EA (electro-absorption) modulator as the intensity modulator, its duty rate is also at most 20 (see Non-patent reference 1). Furthermore, it has a problem that the optical signal-to-noise ratio (OSNR) of the generated data light greatly degrades in return for the improved duty rate.
However, if in order to generate data light with high duty rate, an optical clock is used in place of the CW light source, data light can be generated in accordance with the duty rate of the optical clock.
There is a mode-locking method as one of the technologies for generating an optical clock with high duty rate. In a configuration adopting the mode-locking method, as shown in FIG. 3, duty rate is improved by exciting a standing wave in a resonator composed of a gain medium, a wavelength selector and a modulator. More specifically, there are a mode-locked semiconductor laser and a mode-locked fiber laser as such a light source. For these light sources, see, for example, Non-patent reference 2. However, although the oscillation of data light by these mode locked lasers has an advantage of easily generating optical clocks with high duty rate, it has also a problem that its device configuration becomes complex and its size becomes large. Furthermore, the repetition frequency of an optical clock depends on the length of a resonator, and the length of the resonator fluctuates due to the change of an external environment and the fluctuations of temperature. Therefore, a large-scale stabilization device is indispensable for the stable generation of optical clocks.
So far the prior art for realizing a data light source with high duty rate has been described.                Non-patent reference 1: IEE Electron. Lett., vol. 31, page 671, 1995        Non-patent reference 2: Journal of The Institute of Electronics, Information and Communication Engineers, C-1, J80-C-1, page 70, February 1997.        
As described above, the mode-locked clock light source can generate optical pulses with high OSNR and pulse width of pico-seconds or subpico-seconds. However, it is unrealistic as a light source that generates the above-described optical clock with an arbitrary and accurate frequency, from the practical point of view, such as a manufacturing technology, a product price, a device size and the like. Although the conventional method for applying an optical gate to CW light using an intensity modulator and generating data light can be easily realized by a configuration simpler than a mode locked laser pulse light source, it has a problem that it is difficult to generate optical pulses with high duty rate and the like.
Therefore, it is important to provide a method for generating data light which is an optical pulse with a high OSNR, pulse width of pico-seconds or subpico-seconds without being restricted by the bit rate of data, and Fourier transform limit (TL)(light pulse without frequency chirp), whose bit rate is 10 Gb/s or more and which can accurately and stably operate in arbitrary bit rate.